The present invention relates to a steam valve provided on a steam inlet pipe of a steam turbine for installation in power-station plants and to a steam turbine plant having a steam valve. More particularly, the invention relates to a steam valve constituted by a main steam stop valve having a bypass valve and to a steam turbine plant having such a steam valve.
A steam turbine of the type to be installed in thermal power plants and nuclear power plants is configured as shown in FIG. 5. As FIG. 5 shows, steam generated in the steam generator is supplied to a high-pressure turbine 3 through a main steam stop valve 1 and a governing valve 2. In such a steam turbine, the super high-pressure and super high-temperature steam generated in the steam generator such as a boiler is sectionally supplied to the high-pressure turbine 3 at the start of the steam turbine. At this point, a very large thermal stress develops in any metal component at that part of the turbine 3. The thermal stress deforms the metal component, which may cause cracks and breakage.
In order to suppress such a large thermal stress from developing, so-called full-circumference admission is performed from the start of the steam turbine to the initial loading, thereby warming up the steam turbine, by fully opening the governing valve 2 and controlling the steam flow rate by means of the main steam stop valve 1. This is why the main steam stop valve 1 is configured to control the seam flow rate.
In the example shown in FIG. 5, the steam exhausted from the high pressure turbine 3 is guided to a reheater 31, and then to a medium pressure turbine 33 via a combination reheater valve 32. The rotary shafts of the high pressure turbine 3 and the medium pressure turbine 33 are connected to a power generator 34.
FIG. 6 is a sectional view showing the structure of a main steam stop valve of the conventional type. The main steam stop valve 1 has a valve casing 5 and a valve cover 6, which constitute a pressure vessel and define a valve chamber 4. In the valve casing 5, a baffle plate 7 and a valve seat 8 protrude. The valve chamber 4 contains a strainer 9 and a valve body 10. The valve body 10 is connected to a valve rod 11 and is driven by an oil pressure applied from a hydraulic cylinder 12. Steam S supplied from the steam generator flows through a steam inlet port “I” into the valve chamber 4. The steam “S” passes through the strainer 9 and the valve seat 8 and then flows out from a steam outlet port “O” to the governing valve 2.
FIG. 7 is a sectional view depicting the structure of a valve body of the conventional type. The valve body 10 of the main steam stop valve comprises a cylindrical main valve body 14 and a bypass valve body 15. The bypass valve body 15 can slide in the main valve body 14. An upper end of the bypass valve body 15 projects from the top of the main valve body 14, and a lower end thereof is coupled with the valve rod 11.
An annular wall 16 is formed on that part of the bypass valve body 15, which projects from the top of the main valve body 14. This part of the bypass valve body 15 is closed. A plurality of steam inlet ports 17 are made in the annular wall 16, extend parallel to the direction in which steam flows and lie one above another. The bypass valve body 15 has a steam passage 18 made in the middle part thereof and a steam outlet port 19 made in the lower part thereof. Since the bypass valve body 15 is provided in the main valve body 14, the bypass valve body 15 is configured to adjust the opening of the valve as the valve rod 11 pushes the bypass valve body 15 up against the stream of steam.
As mentioned above, the valve body 10 of the main steam stop valve has the bypass valve body 15 inside the main valve body 14. When the steam turbine is started, the valve body 10 is moved to fully open up the governing valve 2, the main valve body 14 is moved to abut on the valve seat 8 to a fully closed position, and only the bypass valve body 15 is operated to control the steam flow rate. FIG. 7 shows the main valve body 14 of the valve body 10 of the main steam stop valve, which is abutting on the valve seat 8, closing the valve body 10. FIG. 7 also shows the bypass valve body 15 pushed up by the valve rod 11 to the highest position it can take in the main valve body 14. While the bypass valve body 15 remains at the highest position, all steam inlet ports 17 made in the annular wall 16 lie above the top of the main valve body 14, and the bypass valve body 15 is fully opened.
In the main steam stop valve so configured as described above, steam S flows at a considerably high speed into the many steam inlet ports 17 of the bypass valve body 15. The steam S passing through the steam inlet ports 17 made in one side of the bypass valve body 15 and the steam passing through the steam inlet ports 17 made in the other side of the bypass valve body 15 collide with each other in the space defined by the annular wall 16. As a result, the kinetic energy of the steam decreases, and the speed of the steam flow decreases.
Then, the steam at a reduced speed restores the pressure as it passes through the steam passage 18 of the bypass valve body 15. The steam then flows from the main steam stop valve 1 through the steam outlet ports 19 made in the downstream side of the bypass valve body 15. The steam then flows toward the nozzles and vanes of the steam turbine through the governing valve 2 located further downstream side.
The steam flown through the steam inlet ports 17 into the bypass valve body 15 has its kinetic energy reduced and flows at low speed. Therefore, the bypass valve body 15 is not eroded even if it is applied with a trace of drain and oxide contained in the steam.
The bypass valve body 15 described above is called a porous main steam stop valve because it has a plurality of steam inlet ports 17. Such a bypass valve body is disclosed as a structure that prevents damages resulting from erosion, in Japanese Patent Publication No. 61-57442 and Japanese Patent Application Laid-Open Publication No. 2006-46331, the entire contends of which are incorporated herein by reference.
In thermal power plants and nuclear power plants, oxides are formed in the tubes in the steam generators such as boilers and in the steam pipes extending from the steam generators to the steam turbines. At the start of the steam turbines, the oxides contained in the steam flow to the bypass valve body 15 of the main steam stop valve. Particularly, in old plants, oxides are formed in a large amount. The amount of generated oxides increases in concord with the hours the plant has been operated. In other words, the longer the plant has been in service, the larger the amount of oxides formed.
FIG. 8 is a transverse sectional view of the main steam stop valve shown in FIG. 6. As seen from FIG. 8, steam S flowing through the inlet port “I” made in the valve casing 5 flows along the outer circumferential surface of the strainer 9 up to the baffle plate 7 that is opposed to the inlet port I. Since the oxides contained in the influx steam S is heavy, a greater part thereof also flows to the baffle plate 7, by virtue of the inertia of the flow.
Consequently, the oxides pass through the strainer 9, enter inside the strainer 9, and eventually impinge, directly on the outer circumferential surface of the annular wall 16 of the bypass valve body 15. The impingement is prominent, particularly at that part of the annular wall 16 which is indicated by line A in FIG. 8.
As a result, the outer circumferential surface of the annular wall 16 of the bypass valve body 15 is locally eroded with the oxides, at the part indicated by line A in FIG. 8. The steam inlet ports 17 made in this part of the annular wall 16 are deformed. Inevitably, the bypass valve body 15 may fail to perform its function, i.e., the control of the flow rate of steam.